A CONCEPTUAL SNAPSOT OF A BIG COASTAL DUNE AQUIFER: MAGILLIGAN, NORTHERN IRELAND N. S. Robins British Geological Survey, Maclean Building, Wallingford OX10 8BB, UK P. Wilson British Geological Survey, Dundonald House, Upper Newtownards Road, Belfast, BT4 3SB ABSTRACT The Magilligan sand spit dune field is situated on the eastern mouth of Lough Foyle in County Londonderry, Northern Ireland. It is a large triangular-shaped site some 7 km by 4 km by 1.5 km wide (about 800 hectares) and maintains areas, particularly in the eastern part, with slacks that regularly flood in winter. The size of the system acts as a buffer to external drivers due to the large volume of groundwater stored, the longer travel distances and lower hydraulic gradients. However, unlike many other coastal dune sites with humid dune slacks in the British Isles the sand is not wholly underlain by silt and clay, as raised beach sand and gravel deposits are in contact with the sand aquifer in some places. A preliminary water balance suggests that the majority of the discharge from the sand aquifer occurs via the underlying raised beach deposits and only a small amount discharges directly from the sand aquifer beneath the foredunes. Available water level monitoring is skewed towards the wetter end of the dune system; no significant short-term water level trends are apparent. The data also indicate that recharge regularly takes place within the sand aquifer interspersed by periods of groundwater level recession. KEYWORDS Sand dune. Groundwater. Water budget. Dune slack. INTRODUCTION Coastal dune wetlands are widely distributed along the Atlantic, North Sea and Baltic coasts (Doody 2008). They comprise open mobile sand and mature vegetated static dunes. The elevation of the water table within the dunes dictates the morphology of the dune fields and determines the elevation at which wetland dune slacks, the flat-floored depressions that occur within the sand dunes, are formed (Stuyfzand 1993). Humid dune slacks, as defined in the EU Habitats Directive, might have a water table <0.5 below the slack floor in summer and may be flooded during part of the winter. An alkaline environment promotes a rich and diverse assemblage of basiphilous vegetation. A conceptual understanding of the factors controlling the hydrological regime in dune systems is, therefore, fundamental to successful conservation management, as directed by the EU Habitats Directive, to enable prediction of likely future impacts on the dune system ecology. Magilligan is the largest coastal dune system in the United Kingdom and Ireland with an area of about 800 hectares. It is a spit system at the eastern end of Lough Foyle in County Londonderry (Figure 1). The Magilligan Dunes are located on the end of a cuspate foreland. The underlying estuarine clays, and raised beach sands and gravels result from Holocene sea level decline (Carter 1982). The bedrock lithologies beneath the sand have a complex and as yet partly unknown distribution. The system is currently prograding (NIEA 2010) due to the convergence of the Atlantic waves and Lough Foyle waves (Carter 1986). The main foredune faces north onto Magilligan Strand, a 7 km length of gently sloping beach. The dunes are up to 1.5 km wide and the west-facing Foyle estuary coast has a 4 km length of dunes. The dunes have a typical psammosere zonation (Doody 2008) resulting in initial embryo dunes through to scrub and are designated as a Special Area of Conservation (SAC). The highest dunes are up to 7 m above Ordnance Datum. The dunes overlie raised beach sand and gravel as well as estuarine clay and peat to form a large spit at the base of a basalt scarp on top of chalk which in turn overlies Mercia Mudstone. The Mercia Mudstone outcrops to the seaward side of the scarp, concealed only by unconsolidated Holocene sands and clays. The sand is generally about 20 m thick. Exceptionally a borehole in the north west corner of the spit penetrated 25 m sand over 30 m of raised beach sands and gravels and glacial and estuarine clays and gravels (borehole record held at GSNI, Belfast). The estuarine clays have been dated at 7600 years BP implying that the overlying aeolian sands are considerably younger (Carter 1975). The superficial strata rest on Mercia Mudstone Group rocks, neither the basalt nor the chalk outcropping north west of the base of the basalt scarp. Up to 98 m of Holocene material has been proven on Magilligan Plain behind the dune field. The aeolian sands are medium- to fine-grained, are 90% quartz and 8% calcium carbonate (shell debris) with subordinate heavy minerals, mainly magnetite, epidote and biotite (Carter 1975). The sand varies in colour from pale brown to grey. Up to 0.2 m of sandy and fibrous peaty topsoil is present on the slack floors. Water flows off the basalt scarp onto the sands. The sands back onto estuarine flats which are extensively drained. However, the relatively poor transmissive properties of the alluvial strata do not allow groundwater to flow inland into these deposits. There is a main drainage ditch along the dunes and flats divide; this acts as a near constant head feature although little flow is observed in the ditch even when the site is exceptionally wet. No runoff appears to take place other than along a narrow ditch at the foot of the basalt scarp flowing directly to the sea. The sand deposits, therefore, receive only direct rainfall recharge over their full areal extent, no overland flow or groundwater throughflow is likely either from the basalt/chalk scarp or from the alluvial plain to the east. Chemically, the groundwater within the sand is of the Ca-HCO3 type reflecting the abundance of shell debris. The pH ranges from 7.5 to 8.0, while HCO3 <300 mg/l and Cl ranges from 30 to 40 mg/l reflecting the maritime environment of the spit. There are up to seven years of transducer-derived hydrograph data recorded at 1 hour intervals from ten shallow dipwells open to the sand at their base. A total of 14 dipwells are sited for best coverage of a control area situated between a large Ministry of Defence training centre and a caravan park rather than distributed formally about the dune system or along a transect (Figures 2 and 3). They are all situated in slacks where the water table is shallow, none on the adjacent ridges where deeper wells would be needed to access the water table. The part of the dune system used by the Ministry of Defence has restricted public access. Military activity may have both a negative impact on fauna and a positive impact where activity tends to help remobilise the sand. There is a risk of nutrient contamination from the caravan park and from grassland farming which encroaches onto the sand. Magilligan is protected in that there is no public access to much of the dune system. Like other military protected dune zones, such as Pendine in South Wales, it is assumed that the site is as near pristine as possible given natural impacts of dry N deposition and the impact of marine aerosols. Scale has a direct impact on dune status and dune stability because larger sites are better protected against change than smaller ones. This is largely a response to the greater volumes of groundwater stored in the larger sites, longer transport distances and consequent lower hydraulic gradients towards the peripheral discharge zones from the dune system. This concept of relative stability can be demonstrated at Magilligan, a stable site compared to conditions at smaller dune spit systems that also have limited management. The relative hydraulic stability at the larger sites has a direct bearing on the ecohydrology of such dune systems with implications that suggest a need for light-touch conservation management only. Species of ecological importance that occur at Magilligan, for example, include rare plants such as smooth cat‘s ear (Hypochoeris glabra) and hoary whitlow grass (Draba incana) as well as the bee orchid (Ophrys apifera) and marsh helleborine (Epipactis palustris.). Magilligan is the only known site in Ireland which supports Rhytidium rugosum (NIEA 2010). COASTAL DUNE SCALE AND WATER TABLE ELEVATION Most coastal dunes in the British Isles are shallow rain-fed aquifers draining to the periphery of the dune system; to the foredunes and the beach and inland to low-lying land. They are generally underlain by estuarine or glacial clays which form a distinct weakly permeable hydraulic base to the aquifer, i.e. most dune sands are in poor hydraulic continuity with bedrock (Stratford et al. 2013). The groundwater dome within the dune system is controlled by the physical properties of the sand aquifer, the areal dimensions of the dune system and effective precipitation. Each coastal dune system has its own unique features but there are nevertheless common features between all of them (Stratford et al. 2013, Robins et al. 2013). Therefore, the sensitivities of each system may differ, one site might be susceptible to changes in vegetation, another to coastal erosion and accretion, while a third is sensitive to the rainfall regime. In all these systems the ecohydrological status is linked directly to dune hydrogeology (Davy et al. 2006). There is a distinct relationship between the overall size of a dune system and the elevation of the water table within it. Robins et al. (2013) investigated this relationship using observed data from Whiteford Burrows on the Gower Peninsula in South Wales. Whiteford Burrows is 4 km in length and up to 1 km broad backing onto estuarine mud flats. On 17 March 1995 a severe late winter storm event caused extensive erosion of the foredune, reducing the effective width of the dune system by 40 m, i.e.